Research in Progress: Making Artificial (or "Bionic") Vision Look More Like Natural Vision

Researchers from the United States and Europe are attempting to improve the quality of artificial vision, such as the images produced by the Argus II Retinal Prosthesis, also called the "bionic eye."

Their preliminary laboratory research indicates that electrical stimulation of retinal cells can produce the same patterns of activity (or "natural vision") that occur when the retina views a moving object, including the ability to see shape, color, and depth.

[Note: The term "spatiotemporal" relates to both time and space. "Spatial" refers to the relationship of objects within an area or space, while "temporal" refers to a sequence of events occurring in real time.]

The authors are Lauren H. Jepson; Pawel Hottowy; Geoffrey A. Weiner; Wladyslaw Dabrowski; Alan M. Litke; and E.J. Chichilnisky, who represent the following institutions: Salk Institute for Biological Studies, La Jolla, CA; University of California, San Diego; AGH University of Science and Technology, Krakow, Poland; University of California, La Jolla; University of California, Santa Cruz; and Stanford University, Stanford, CA.

About the Research

In laboratory tests, researchers have used electrical stimulation of retinal cells to produce the same patterns of activity that occur when the retina sees a moving object. Although more work remains, this is a step toward restoring natural, high-fidelity vision to blind people, the researchers say.

Just 20 years ago, "bionic" vision was more a science fiction cliché than a realistic medical goal. But in the past few years, the first artificial vision technology has come on the market in the United States and Western Europe, allowing people who've been blinded by retinitis pigmentosa to regain some of their sight. While remarkable, the technology has its limits. It has enabled people to navigate through a door and even read headline-sized letters, but not to drive, jog down the street, or see a loved one's face.

The Retina and Artificial Vision

The retina contains several cell layers. The first layer contains photoreceptor cells, which detect light and convert it into electrical signals. Retinitis pigmentosa and several other blinding diseases are caused by a loss of these cells. The strategy behind many bionic retinas, or retinal prosthetics, is to bypass the need for photoreceptors and stimulate the retinal ganglion cell layer, the last stop in the retina before visual signals are sent to the brain.

Current technology does not have enough specificity or precision to reproduce natural vision, he said. Although much of visual processing occurs within the brain, some processing is accomplished by retinal ganglion cells. There are 1 to 1.5 million retinal ganglion cells inside the retina, in at least 20 varieties. Natural vision – including the ability to see details in shape, color, depth, and motion – requires activating the right cells at the right time.

Parasol Cells and Artificial Vision

The new study shows that patterned electrical stimulation can do just that in isolated retinal tissue. The [researchers] focused their efforts on a type of retinal ganglion cell called parasol cells. These cells are known to be important for detecting movement, and its direction and speed, within a visual scene. When a moving object passes through visual space, the cells are activated in waves across the retina.

The researchers placed patches of retina on a 61-electrode grid. Then they sent out pulses at each of the electrodes and listened for cells to respond, almost like sonar. This enabled them to identify parasol cells, which have distinct responses from other retinal ganglion cells. It also established the amount of stimulation required to activate each of the cells.

Next, the researchers recorded the cells' responses to a simple moving image – a white bar passing over a gray background.

Finally, they electrically stimulated the cells in this same pattern, at the required strengths. They were able to reproduce the same waves of parasol cell activity that they observed with the moving image.

One Type of Artificial Vision: the Argus II Retinal Prosthesis

The Argus II has been approved to treat adults with severe to profound retinitis pigmentosa (RP), a rare, inherited degenerative disease that damages light-sensitive cells in the retina, resulting in decreased vision at night or in low light; loss of side (peripheral) vision; and loss of central vision as the disease progresses. At present, there is no cure for RP.

The Argus II device consists of the following components:

a small video camera

a transmitter mounted on a pair of eyeglasses

a video processing unit (VPU)

an artificial retina (the implanted retinal prosthesis, which is an array of electrodes)

The video camera images are transformed into electronic data by the VPU that sends signals to a wireless receiver implanted in the eye (the retinal prosthesis electrodes). The electrodes allow the electronic signals to bypass the damaged retina and transmit directly to the brain, where they are interpreted as visual images.

More about the Study from Neuron

Natural vision relies on spatiotemporal patterns of electrical activity in the retina. We investigated the feasibility of … reproducing such patterns with epiretinal [i.e., overlying the retina] prostheses.

Multielectrode recordings and visual and electrical stimulation were performed on populations of identified ganglion cells in isolated peripheral primate retina. Electrical stimulation patterns were designed to reproduce recorded waves of activity elicited by a moving visual stimulus.

Electrical responses in … parasol cells exhibited high spatial and temporal precision, matching or exceeding the precision of visual responses measured in the same cells.

These results suggest the possibility of producing rich spatiotemporal patterns of retinal activity with a prosthesis and that temporal multiplexing [i.e., combining multiple message signals or data streams into one signal] may aid in reproducing the neural code of the retina.

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